High-tension electrical switches are operated in the following manner: (1) a spring operating mechanism is charged either by manual force on a crank or by a motorized charger; (2) the energy stored in the springs is released by a tripping mechanism, typically either solenoid trip mechanisms or automatic release pawls that trip the operating mechanism at the end of the charging stroke; then (3) the released energy of the spring mechanism is transferred to the switch, causing the switch to open or close rapidly. The operation of a typical high-tension switch requires the precise transmission of bursts of torsional energy from the spring operating mechanism to the switch. In order to achieve this, most switches couple the spring operating mechanism directly to the switch. High-tension electrical switchgear components are therefore normally mounted within a metal cabinet which holds the operating mechanism, the switch, fuses, and other instrumentation.
Especially when the high-tension switchgear is motorized, it is important that maintenance personnel be able to repair and test the operating mechanism without actually opening or closing the switch. A commonly used system which allows maintenance on the operating mechanism and its accessories without disturbing the switch is a "draw-out" system. This operating system, usually including a motorized charger and which may include solenoid trippers, is mounted on a drawer. The system is configured so that the operating mechanism disconnects from the switch when the drawer is pulled out from the front of the cabinet. The draw-out system has a number of disadvantages. Since the draw-out is side-mounted, it requires additional cabinet width to house the drawer. Additionally, while the operating mechanism is drawn out, there is no complete seal between maintenance personnel and the energized area in the cabinet. Most importantly, the arrangement for disconnecting and reconnecting the mechanical linkage between the operating mechanism and the switch as the drawer is withdrawn for maintenance or replaced, is located within the cabinet where maintenance is difficult and the disconnecting and reconnecting system is frequently a source of trouble.
Another system for allowing access to the switch operating mechanism while the switch is energized is the side-mount system. This system provides an operating mechanism which is mounted on the outside of the sidewall of the cabinet. A second unit, with the operating mechanism mounted on the exterior of the side wall of the cabinet is paired with the first so that the sides with the mounted operating mechanisms are facing each other, and a common maintenance access space is provided between the cabinets. The common maintenance access space may also be enclosed in a cabinet. Although this system is simpler and less expensive than the draw out system, its large lateral footprint results in increased operating costs because of the additional plant floor space requirements. Additionally, it requires that maintenance personnel work in the cramped and dangerous maintenance access space between the energized areas of the paired cabinets.